1. Field of the Invention
The present invention is directed to a power supply including an inverter to provide a high frequency AC voltage from a DC voltage source for driving a load, for example, a discharge lamp.
2. Description of the Prior Art
Power supplies with an inverter have been well known in the art as disclosed, for example, in U.S. Pat. No. 5,130,610; U.S. Pat. No. 4,461,980; U.S. Pat. No. 5,138,234; and U.S. Pat. No. 4,719,556.
U.S. Pat. No. 5,130,610 discloses a typical power supply including an inverter providing a high frequency AC voltage from a DC voltage. As shown in FIG. 1 of the attached drawings, this prior power supply has a diode-bridge rectifier DB providing from an AC voltage source AC a rectified DC voltage which is smoothed by a smoothing capacitor C0 to give a smoothed DC voltage to the inverter. The inverter INV includes a pair of transistors Q1 and Q2 which are connected in series with the smoothing capacitor C0. Connected across the first transistor Q1 is a series L-C resonant circuit composed of an inductor L1 and a capacitor C1 which are connected in series with a coupling capacitor C2. A transformer T is connected in circuit with a primary winding T1 connected across the capacitor C1 and with a secondary winding T2 connected to a load LD. The transistors Q1 and Q2 are controlled by a driver circuit to alternately turn on and off at a high frequency to cause the L-C resonant circuit to produce an oscillating voltage across the primary winding T1 to thereby apply a corresponding AC voltage to the load LD. Thus, the L-C resonant circuit provides a predetermined high frequency AC voltage to drive the load as long as the load is connected to the inverter in a good condition. However, when a load is detached to render the power supply to operate under no load condition, or when the load is deteriorated to have increased impedance as is frequently seen in the life end of the discharge lamp, the L-C resonant circuit responds to develop an excessive high voltage. With this result, the inverter sees an abnormally high resonant current flowing through the transistors to eventually destroy the transistors or at least severely damage the transistor with a considerably increased loss.
In order to eliminate the above problem, U.S. Pat. No. 4,461,980 proposed to incorporate a protection circuit which disables the inverter when the load is detached. As shown in FIG. 2 of the attached drawings, the inverter INV comprises two transistors Q1 and Q2 in half-bridge configuration with capacitors C2 and C3, and a series L-C resonant circuit composed of an inductor L1 and a capacitor C1 which are connected across the output of the half-bridge. Transistors Q1 and Q2 are coupled to a transformer T3 so as to be driven to turn on and off in a self-excited manner. A load, which is a discharge lamp, is connected across the capacitor C1. The protection circuit comprises a pair of series connected diode D3 and D4 is connected in parallel relation to the series combination of capacitor C2 and C3, and a bimetallic switch B with a heater H connected in a base-emitter path of transistor Q2. The heater H is inserted in a line connecting a tap point TP of the inductor L1 to a connection point between diodes D3 and D4. When the lamp is detached, the L-C resonant circuit responds to produce an increased voltage with an attendant increase in a voltage developed across the inductor L1, which allows a sufficient current to flow the heater H, thereby actuating the bimetallic switch B to shunt the base-emitter path of transistor Q2. In this manner, the inverter ceases operating upon detachment of the lamp, thereby preventing continued oscillation of the L-C resonant circuit and therefore well protecting the transistors from excessive high voltage which would be otherwise produced.
U.S. Pat. No. 5,138,234 discloses another prior art power supply which includes a limiter capable of avoiding excessive high voltage developed in an inverter when a load is detached therefrom. As schematically reproduced in FIG. 3 of the attached drawings, the inverter is energized by a fixed DC voltage source E and comprises a pair of transistors Q1 and Q2 arranged in a half-bridge configuration with a pair of capacitors C2 and C3. A series L-C resonant circuit composed of an inductor L1 and a capacitor C1 is connected to the bridge output so as to produce a high frequency AC voltage in response to turn on and off of transistors. An output transformer T is in circuit with a primary winding T1 connected across the capacitor C1 and with a secondary winding T2 connected to a lamp. The limiter comprises a pair of series connected clamping diode D3 and D4 which are connected across the DC voltage source E in parallel with the series combination of the capacitors C2 and C3. Connected to a point between the diodes D3 and D4 is a center tap TP of the primary winding T1 so that the connection point between diodes D3 and D4 is clamped at a fixed voltage level corresponding to the input voltage from the DC voltage source E to the inverter. With this result, if the L-C resonant circuit tends to produce an excessively high voltage upon detachment of the lamp from the inverter with an attendant voltage increase at the connection point, thus increased voltage is permitted to return through the primary winding T1, diodes D3 and D4 into capacitors C2 and C3, thereby avoiding the L-C resonant circuit from producing the excessive high voltage.
U.S. Pat. No. 4,719,556 discloses a further prior art power supply which includes a limiter for the same purpose as above. As shown in FIG. 4 of the attached drawings, the power supply comprises a like half-bridge circuit of transistors Q1 and Q2 and capacitors C2 and C3, and a series L-C resonant circuit of an inductor L1 and a capacitor C1. An output transformer T is in circuit with a primary winding T1 connected across the capacitor C1 and with a secondary winding T2 connected to a load. The limiter comprises a pair of series connected diodes D3 and D4 which is connected across a DC voltage source E in parallel relation to the series pair of capacitors C2 and C3. The diodes D3 and D4 are connected at point between the inductor L1 and the capacitor C1 in order to clamp a voltage at the connection point at a level corresponding to that of the DC voltage source E. Also with this limiter, it is possible to avoid excessive voltage from being produced at the L-C resonant circuit when the load is detached or varies its impedance by a large extent.
However, problems still remain in these prior power supplies in that the provision of the tap to the inductor or the transformer incurs rather complicated assembly with an additional manufacturing cost, as seen in the circuit of FIGS. 2 and 3, and also in that a clamping current will flow through the inductor or output transformer to raise the temperature thereof, as seen in the circuit of FIGS. 2 and 3. Further, in the circuit of FIG. 4 where the clamping diodes D3 and D4 are connected to the point between the inductor L1 and the capacitor C1 forming the L-C circuit, it is impossible to adjust a clamping voltage to which the output of the inverter is limited while assuring to produce a desired oscillation voltage at the L-C circuit. That is, the output of the inverter is limited only to a fixed clamping voltage which poses a strict limitation in the circuit design.